The present invention relates to a crystal puller and method for growing single crystal semiconductor material, and more particularly to a crystal puller and method for growing an ingot or crystal with desired defect characteristics.
In recent years, it has been recognized that a number of defects in single crystal silicon form in the crystal puller (sometimes referred to as a hot zone) as the ingot cools from the temperature of solidification. More specifically, as the ingot cools intrinsic point defects, such as crystal lattice vacancies or silicon self-interstitials, remain soluble in the silicon lattice until some threshold temperature is reached, below which the given concentration of intrinsic point defects becomes critically supersaturated. Upon cooling to below this threshold temperature, a reaction or agglomeration event occurs, resulting in the formation of agglomerated intrinsic point defects.
The type and initial concentration of these intrinsic point defects in the silicon are determined as the ingot cools from the temperature of solidification (i.e., about 1410° C.) to a temperature greater than about 1300° C. (i.e., about 1325° C., 1350° C. or more); that is, the initial type and initial concentration of these defects are controlled by the ratio v/G0, where v is the growth velocity and G0 is the average axial temperature gradient over this temperature range. In general, a transition from self-interstitial dominated growth to vacancy dominated growth occurs near a critical value of v/G0 which, based upon currently available information, appears to be about 2.1×10−5 cm2/sK, where G0 is determined under conditions in which the axial temperature gradient is constant within the temperature range defined above. Accordingly, process conditions, such as growth rate (which affect v), as well as hot zone configurations (which affect G0), can be controlled to determine whether the initial intrinsic point defects within the silicon single crystal will be predominantly vacancies (where v/G0 is generally greater than the critical value) or self-interstitials (where v/G0 is generally less than the critical value).
Defects associated with the agglomeration of crystal lattice vacancies, or vacancy intrinsic point defects, include such observable crystal defects as D-defects, Flow Pattern Defects (FPDs), Gate Oxide Integrity (GOI) Defects, Crystal Originated Particle (COP) Defects, and crystal originated Light Point Defects (LPDs), as well as certain classes of bulk defects observed by infrared light scattering techniques (such as Scanning Infrared Microscopy and Laser Scanning Tomography). Also present in regions of excess vacancies are defects which act as the nuclei for the formation of oxidation induced stacking faults (OISF). It is speculated that this particular defect is a high temperature nucleated oxygen precipitate catalyzed by the presence of excess vacancies.
Defects associated with the agglomeration of silicon self-interstitial atoms include such observable crystal defects as A-defects and B-defects (sometimes referred to as A-type swirl defects and B-type swirl defects). A-defects have been reported to be interstitial-related dislocation loops. B-defects have been reported to be three-dimensional interstitial agglomerates.
In addition to the point defects which exist as solutes in the monocrystalline silicon, many impurities such as dopants and oxygen also exist as solutes in Cz silicon and may affect the formation of agglomerated intrinsic point defects (e.g., A-, B-, and D-defects and OSF nuclei and OSF) or even co-agglomerated with intrinsic point defects. Agglomerated defects exist as separate phases in the Cz-silicon and can include D-defects, A and B-defects, OSF nuclei and OSF, oxides, nitrides, silicides and other precipitates. Formation and distribution of agglomerated defects are functions of growth conditions at the melt/crystal interface, and time-temperature (or thermal) history of each location in the Cz-silicon crystal.
Referring to FIG. 3, the formation of agglomerated defects involves various physical and chemical processes. However, in a simplistic sense, it is possible to identify a set of rate controlling steps in a given temperature range within the crystal. For example, one can identify several significant steps in formation of agglomerated defects and the temperature range in which each plays a dominant role. These steps include:                1) Incorporation of point defects: involves establishment of a new point defect distribution very close to the melt/crystal interface by interplay between diffusion and recombination of point defects. It has been shown that by controlling the crystal growth rate (in average sense, the crystal pull-rate, v) and the magnitude of the axial temperature gradient (Gs,f,z) in the crystal at the melt/crystal interface, the initial point defect type and concentration within a short distance from the interface can be controlled.        2) Out-diffusion and recombination: During this phase intrinsic point defects (silicon self interstitial atoms and/or crystal lattice vacancies) may out-diffuse to the crystal surface or silicon self interstitial atoms and crystal lattice vacancies may diffuse towards each other and recombine mutually annihilating each other.        3) Nucleation: Nucleation (broadly, formation) takes place upon sufficient supersaturation of the dominant point defect. The agglomeration of vacancies generally occurs at temperatures ranging from about 1273 K to about 1473 K, from about 1298 K to about 1448 K, from about 1323 K to about 1423 K, or from about 1348 K to about 1398 K. Controlling the rate of cooling over this temperature range influences the density of agglomerated vacancy defects. The agglomeration of silicon self-interstitials generally occurs at temperatures ranging from about 1373 K to about 1073 K or from about 1323 K to about 1173. The temperature at which predominant nucleation of vacancies takes place decreases with decreasing incorporated vacancy concentration. In other words, lower the vacancy concentration, lower the nucleation rate and lower the temperature at which nucleation occurs.        4) Growth: Growth of stable nuclei follows nucleation.        5) Oxygen precipitation: Oxygen can nucleate in the presence of vacancies and gradually grow between 1323 K-973 K. Oxygen precipitation is enhanced in the presence of vacancies. That is, crystal lattice vacancies and oxygen interstitial atoms may co-agglomerate to form oxygen precipitate nuclei or if formed sufficiently large, oxygen precipitates.        6) Impurity precipitation: Other impurities can play a role in precipitation as well. The temperature range for this step depends on the type and concentration of the impurity.        
FIG. 4 is a schematic of a growing crystal depicting the sequential nature of defect dynamics within the crystal. In sequence, a crystal segment undergoes initial point defect incorporation (I), diffusion and recombination (DR), nucleation (N) and growth (G). Oxygen precipitation (OP) occurs during nucleation and growth. It is evident that the temperature gradient in the crystal at the melt/crystal interface and the crystal growth rate play significant roles in initial point defect incorporation. Subsequent processes, such as nucleation and growth are influenced by the local cooling rates, i.e., the thermal history of the crystal subsequent to the initial incorporation of the intrinsic point defects. During growth, the local cooling rate is given by v×Gs,f,z, where Gs,f,z is the local temperature gradient. Thus, the temperature profile in a crystal is important for the control of the nucleation rate and growth of all precipitates.
In many applications, it is preferred that a portion or all of the silicon crystal which is subsequently sliced into silicon wafers be substantially free of agglomerated defects. There are several approaches for growing defect-free or defect-controlled silicon crystals. In one approach, the ratio v/Gs,f,z is controlled to determine the initial type and concentration of intrinsic point defects. The subsequent thermal history is controlled to allow for prolonged diffusion time to suppress the concentration of intrinsic point defects and avoid the formation of agglomerated intrinsic point defects in a portion or all of the crystal. See, for example, U.S. Pat. Nos. 6,287,380, 6,254,672, 5,919,302, 6,312,516 and 6,328,795, the entire disclosures of which are hereby incorporated herein by reference. In another approach, sometimes referred to as a rapidly cooled silicon (RCS) growth process, the ratio v/Gs,f,z is controlled to determine the initial type and concentration of intrinsic point defects. The subsequent thermal history is controlled to rapidly cool the crystal through a target nucleation temperature to avoid the formation of agglomerated intrinsic point defects. This approach may also include allowing prolonged cooling above the nucleation temperature to reduce the concentration of intrinsic point defects prior to rapidly cooling the crystal through the target nucleation temperature to avoid the formation of agglomerated intrinsic point defects. See, for example, International Application No. PCT/US00/25525 published on Mar. 29, 2001 under International Publication No. WO 01/21861, the entire disclosure of which is incorporated herein by reference. In a similar approach, the growth conditions, v/Gs,f,z and cooling rate through the target nucleation temperature are controlled in order to limit the size, and in some cases the density, of vacancy-related agglomerated defects, and optionally the residual vacancy concentration, in single crystal silicon wafers derived therefrom. See, for example, PCT Application Serial No. PCT/US02/01127 published on Aug. 29, 2002 under International Publication Number WO 02/066714, the entire disclosure of which is hereby incorporated herein by reference.
However, depending on the application in which the silicon will be used, it may be acceptable or even desirable to produce silicon having any of the above described defects. That is, it may be acceptable or desirable to produce material a portion or all of which contains either D-defects, OSF, OSF nuclei, B-defects or A-defects or combinations thereof. For example, in some applications, silicon crystals are grown under conditions wherein D-defects form throughout the crystal. Silicon wafers sliced from such D-defect containing crystals may then be subjected to thermal anneals to remove the D-defects from the surface region of the wafer, or subjected to an epitaxial deposition process wherein the D-defects revealed on the surface of the wafer as COPs are filled by the deposition of an epitaxial layer on the surface of the wafer. In other applications, it may be desirable to grow a crystal under conditions wherein B-defects form throughout the crystal. Silicon wafers sliced from such B-defect containing crystals may be subjected to rapid thermal anneals to dissolve the B-defects. See, for example, International Application No. PCT/US/00/25524 published on Mar. 29, 2001 under International Publication No. WO 01/21865.